1. Field of the Invention
This invention relates to an electronic controlled carburetor for use in internal combustion engines and, more particularly, to such a carburetor with feedback means responsive to selected constituents of the exhaust gases to ensure that the fuel supplied to the engine is correct to maintain a desired optimum air/fuel ratio.
2. Description of the Prior Art
It is the common practice to minimize the emission of pollutants to the ambient by using a three-way catalyst, located within the stream of exhaust gases, which catalyzes the oxidation of hydrocarbons (HC) and carbon monoxide (CO) and also the reduction of oxides of nitrogen (NOx). It has been discovered that a difficulty with such a three-way catalyst system is that if the fuel metering is too rich, the NOx will be reduced effectively, but the oxidation of CO will be incomplete. On the other hand, if the fuel metering is too lean, the CO will be effectively oxidized but the reduction of NOx will be incomplete. In order to make such a three-way catalyst system operative, it is necessary to have very accurate control over the fuel metering function to maintain a stoichiometric air fuel ratio.
It has been therefore proposed to employ an electronic controlled carburetor with feedback means responsive to the presence of selected constituents of the engine exhaust gases to ensure that the fuel supplied to the engine is correct to maintain a desired optimum air/fuel ratio such as stoichiometric. The feedback means was employed to alter the average degree of opening of a solenoid valve located in an air bleed passage means to adjust the rate of air flow introduced into a fuel passage means communicating between a fuel reservoir chamber and a carburetor induction passage. The feedback means comprises a control circuit for providing a drive pulse signal the duty ratio of which is dependent upon the selected constituents of the engine exhaust gases to the solenoid valve which thereby opens and closes the air bleed passage means with the required ratio of its "open" time to its "close" time. The solenoid valve is required to have a large dynamic range to control the air/fuel ratio of the mixture supplied to the engine over all engine operating modes. However, if the dynamic range is too large pulsations will occur in the fuel discharged into the carburetor induction passage upon the opening and closing of the solenoid valve operating with a drive pulse signal, causing air/fuel ratio variations and engine torque fluctuations which would eventually leads to engine hunting.
If the solenoid valve has its dynamic range limited in order to avoid the discharging fuel pulsation problem, the range of effective air/fuel ratio control of the solenoid valve will be restricted. For example, the solenoid valve will remain open when the vehicle runs at high altitudes and the ambient air density is low.
Accordingly, the invention as disclosed is directed generally to the solution of the above and related problems and more specifically to means permitting the solenoid valve to operate around 50% duty ratio of its "open" time to its "close" time substantially over all engine operating modes.